The present invention relates to hot dip coating of steel strip with molten metal, such as, zinc. More particularly, the present invention relates to removing and recovering bottom dross from the molten metal, such as, zinc during galvannealing or galvanizing.
In some galvannealing and galvanizing processes, a dross is formed in the liquid metal bath. The dross that has the specific gravity higher than that of the molten metal (i.e., bottom dross) gradually sinks and deposits at the bottom of the bath. Once a significant amount of bottom dross is accumulated, turbulence caused, for example, by the steel strip passing through the bath can dislodge particles of the accumulated dross bottom and bring it in contact with the steel strip. The dross deposited on the surface of the steel strip can cause serious quality problems in the form of dents, irregularities and non-uniform appearance of the product.
For example, bottom dross forms during processes of galvannealing of steel strip with zinc in the presence of less than about 0.15 percent by weight of aluminum. Aluminum is added to molten zinc in order to facilitate adhesion of the zinc to steel. When the concentration of aluminum in the molten zinc is above about 0.15 percent, the thermodynamic equilibrium of the system does not produce bottom dross. However, for efficient galvannealing the level of aluminum should be less than about 0.15 percent by weight, generally less than 0.13 percent by weight. At those lower concentrations, the thermodynamic equilibrium produces an intermetallic Zn--Fe phase. The predominant intermetallic phase is the delta phase (FeZn.sub.7). However, zeta phase FeZn.sub.13 is also often present. Delta and Zeta phases are small crystalline aggregates that have specific density slightly higher than that of the molten zinc. Accordingly, the dross slowly descends to the bottom of the pot and accumulates these during the production campaign. Once significant amounts of bottom dross accumulate in the pot, the turbulence periodically dislodges particles of the dross and brings them in contact with the surface of the steel strip passing through the pot. When the dislodged dross deposits on the surface, it detrimentally effect the quality and the appearance of the galvannealed steel. The areas containing dross create irregularities and protrusions that are unsuitable for use in the production of exterior automotive body panels and the like. Additional quality and maintenance problems are caused by the dislodged dross attaching to and accumulating on the rollers that transport the steel strip through the pot.
To avoid these problems, various methods have been devised to periodically remove bottom dross from the pot. These methods include manual removal of the accumulated dross by specially designed tools, such as a clamshell, a back hoe and a scooper. These manual methods of removing bottom dross are not satisfactory for several reasons. First, the galvannealing operation must be shut down during the removal. The shut down causes significant operating losses. Second, the clean up is time consuming and labor intensive because colloidal crystalline bottom dross is difficult to capture. Third, the attempts to capture dross causes turbulence which resuspends some of it. Finally, only a limited amount of the dross can actually be captured and removed during each clean-up operation. Accordingly, the uncaptured dross gradually accumulates causing an increasing frequency of interruptions of production for dross removal.
To overcome the problems inherent in the manual methods of removal of dross, attempts have been made to use a pump to pump molten zinc from spaces where dross accumulated to outside the pot. However, implementation or commercialization of pumping proposals face formidable problems primarily associated with the difficulties in handling molten metal. For example, Japanese patent application 2-141563 discloses a pump for removing bottom dross from a dip plating cell. Dross is pumped from the bottom of the plating cell and passed through a filter to separate the dross from molten zinc. Since the pump and filter are positioned external of the plating cell, a long stationary suction pipe extends from the plating cell to the pump. Japanese patent application 6-41705 discloses a pump for removing bottom dross from a dip galvanizing vessel. The dross is pumped from the bottom of the vessel and passed through a settling box where the dross is separated from molten zinc. The pump is positioned external of the galvanizing vessel and a long stationary suction pipe extending into the galvanizing vessel is required. The settling box is positioned within the gralvanizing vessel. Positioning of the pump eternal of the coating pot is impractical because priming of the pump with liquid zinc is necessary but very difficult by practical means. This also requires a long suction pipe extending into the coating pot. A long pipe tends to become plugged as the result of chilling, if the bottom dross or molten zinc is being withdrawn too slowly through the pipe. Even if the suction pressure of the pump is adequate, bottom dross would only be locally removed in the vicinity of the inlet of the pipe since the pipe is stationary within the bath and bottom dross tends to remain agglomerated in various locations on the bottom until dispersed by high bath current.
Japanese patent application 63-69956 discloses a pump for removing suspended dross from a dip plating cell. The pump is positioned at about mid-depth within the plating cell and pumps coarse dross through a ceramic filter for separating the dross from molten metal positioned within an external pot. However, this approach only removes floating dross. The dense bottom dross that sinks to the bottom of the plating cell is not removed.
Chemical methods of removal of dross involve increasing aluminum concentration in the molten zinc to convert bottom dross to top dross has undesirable quality consequences as well. Chemical conversion of the dross during galvanneal production can not be accomplished as the amount of aluminum necessary to create the conversion of bottom dross to top dross is excessive for good quality galvanneal. Chemical conversion during galvanized production results in poor coating quality because bottom dross particles floating to the top often adhere to the surface of the strip causing dross pimples or bumps.
Accordingly, the long-felt need for efficient, reliable, and complete removal of bottom dross from a coating pot without interrupting or adversely affecting the coating operation remains unsatisfied.
Thus, one object of the present invention is to provide an efficient and reliable method and apparatus for removing bottom dross from a coating pot to insure good coating quality of steel strip emerging from the pot by mining dross particles adhered to the surface of the strip.
Another object of the present invention as to provide a method and apparatus for removing bottom dross without interrupting the coating operation
A further object of the present invention is to remove bottom dross that accumulates in large areas at the bottom of the vessel.
Other objects of the present invention will become apparent to those skilled in the art upon studying this disclosure and the appended claims.